1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly, to a semiconductor memory device that masks data by controlling a column select line signal in order to deactivate a specific column select line signal when one or more columns are selected, and to a column decoder.
2. Description of the Related Art
A semiconductor memory performs a writing operation for storing data input from an outside source and a reading operation for retrieving data stored in the memory cell. When performing writing and reading operations, not all the data are written and read. For example, if there is no need to change the data of a specific memory cell at a specific timing, such a cell is masked out during the writing operation to prevent data from being written over.
In a conventional semiconductor memory device, when k columns are selected out of n columns and m masking signals of k masking signals M.sub.0 .about.M.sub.k-1 are activated, m data input/output drivers of k data input/output data drivers I/O DRV.sub.0 .about.I/O DRV.sub.k-1 are disabled, so data is simultaneously written to only (k-m) memory cells.
However, in the conventional semiconductor memory devices it is difficult to control the period during which the m data input/output drivers of k data input/output data drivers I/O DRV.sub.0 .about.I/O DRV.sub.k-1 are disabled, and it is difficult to control the timing when the data input/output lines connected to the disabled m data input/output drivers are precharged or equalized to a predetermined voltage level. Also, the conventional semiconductor memory device requires k data input/output lines and k data input/output drivers, which increases the chip size.